1. Field of the Invention
This invention relates to the field of electrical systems and meters designed to measure electrical potential difference and more specifically to instruments designed to interpret potential difference as a relative measure of the effectiveness of sacrifical anodes in protecting structures from corrosion, such structures typically being ships in salt water.
2. Prior Art
In order to protect the hull of a ship from electrochemical corrosion, it is common practice to attach to the hull of the ship a material which has an electrochemical reaction potential more negative than that of the material of which the hull is constructed. In this manner, the material attached to the hull will give off electrons more readily than the hull thereby protecting the hull from corrosion. Such systems are commonly referred to as sacrificial cathodic protection systems. When the sacrificial material is corroded to an extent that it can no longer effectively protect the hull from corrosion, the material is replaced.
The present invention is utilized to detect the point at which the sacrificial anodes should be replaced. This instrument, however, is not limited to use in conjunction with ships.
Corrosion engineers typically use one of the three standard half cell reference voltages when measuring potential difference in corrosion systems. For land-based structures protected by sacrificial anodes, the standard reference is the copper-copper sulfate half cell. For the scientific experimental or laboratory situation, the reference voltage is that of a saturated calomel half cell (calomel is mercurous chloride). The silver-silver chloride cell is the standard reference half cell used for corrosion measurements relating to sea-based installations (such as ships).
Because of the chronology of historical development, all of the prior art instruments have been constructed specifically for use in one of the above three environments. An instrument designed for use in one environment could not conveniently be used in another environment. It is therefore an object of the present invention to provide an instrument which can be used to measure the relative effectiveness of sacrificial cathodic systems as conveniently in one environment as in either of the others.
All electrical instruments designed to measure potential difference have two terminals. In prior art instruments designed to evaluate the relative effectiveness of a sacrificial anode system, one terminal is attached to one of the three standard reference half cells. In these prior art devices, the reference half cells always had a more positive electrochemical reaction potential than the hull of a ship. Thus the flow of electrons in such devices would be from the hull of the ship through the instrument to the reference half cell. Due to the small surface area of the reference half cell, it would become polarized quite quickly to a level which would interfere with accurate measurements.
The device of the present invention uses as its reference half cell a probe of magnesium alloy AZ63 (magnesium, 6% aluminum, 3% zinc, 0.2% manganese and traces of impurities), which has an electrochemical reaction potential which is more negative than that of the hull, thereby causing the electrons to flow from the half cell through the meter to the hull of the ship. The number of electrons which in the prior art devices would flow to the reference half cell, and polarize it such that accurate measurements could not be made, are now caused to flow to the ship where they are distributed over its much larger surface area. The effect of the electrons on such a large surface area is negligible and does not interfere with the accuracy of readings. This allows readings to be taken for much longer time periods without loss of accuracy as compared with prior art devices.
The other terminal of the present invention is used for making the electrical connection between the meter and the structure being tested, for example the hull of a ship. In the present invention, this terminal terminates in a body of metal having a sharp point and generally shaped like an ice pick so that the body of metal is more readily forced through any layers of paint or of barnacles which may be adhering to the hull and make a good electrical connection with the hull.
The prior art devices used half cells of silver-silver chloride, copper-copper sulfate and calomel (mercurous chloride). Each of those half cells has a brittle exterior layer which is easily shattered upon impact. The half cell used in the present invention is a homogeneous alloy of magnesium, having no brittle exterior layer.
The magnesium alloy also acts as an anode and thus will repel any slight coating of oil which it may receive when immersed in the water of a typical harbor. The devices of the prior art have an electron flow in the opposite direction and will not repel these coatings of oil, which tend to interfere with the accuracy of measurements.
The magnesium alloy half cell may also be cleaned as required by a wire brush similar to that used to clean battery terminals.
These and other objects and advantages of the present invention will become apparent as the description is read in conjunction with the drawings.